Adobe - Photoshop Creative Suite Series (CS, CS2, CS3)

Photoshop is probably the best image manipulation program on the planet.  You can do just about anything you want to a photo with Photoshop.  The learning curve isn't bad for basic stuff, but since the program is so powerful, it definitely takes some time to master.  Even masters at the program learn new tricks and techniques all the time.

I highly recommend learning this program for anybody serious in photography, web design, or art.  If you can't afford the latest creative suite version, then I would recommend buying Photoshop Elements.  It does most of what CS does, minus stuff that printing professionals have to have like CMYK editing instead of RGB.